1. Field of the Invention
This invention relates to an improved method for vulcanizingly bonding copper, copper alloys or other metals coated therewith to a rubber composition combined with sulphur and/or organic sulphur compounds.
2. Description of the Prior Art
Various rubber articles are generally made of rubber composition and other materials. For instance tires, belts, hoses and resilient mounting are made of rubber and fibrous materials and/or metallic materials. The qualities or life of such articles are considerably affected by properties which are held by each of said materials and interrelated characteristics such as adhesiveness between said materials when the rubber articles have been produced.
It has been known, in making the rubber article reinforced, for instance, with organic fibers, that moisture involved in the fibers and/or the rubber composition causes deterioration of the organic fibers, above all the lowering of strength due to heating in a high moisture atmosphere during the vulcanization of the rubber composition together with said organic fibers.
It has been found according to U.S. Pat. No. 3,671,478 that adding boric oxide (B.sub.2 O.sub.3) to the rubber composition as a desiccant is very effective for preventing deterioration of the organic fibers, principally the lowering of the strength thereof caused by heating in a high moisture atmosphere, as compared with the effectiveness obtained by using the conventional desiccant such as calcium oxide (CaO). It is disclosed therein that lowering of the adhesion between the rubber and organic fibers can also be prevented thereby. This is understood to be due to the fact that the moisture in the organic fibers and/or the rubber composition is chemically combined with boric oxide contained therein as a desiccant so as to prevent a layer (in said U.S. Patent resorcin-formaldehyde-latex is used for said layer) from being swollen and subjected to hydrolysis. In such method of adding a desiccant such as B.sub.2 O.sub.3, CaO to the rubber composition, however, there is a problem that the greater the moisture content therein, the greater the amount of desiccant which must be added and the addition thereof tends to make the rubber composition itself more moisture absorptive.
As for rubber articles reinforced with metallic material, there would be substantially no problem of deterioration due to heating in the high moisture atmosphere different from those reinforced with the organic fibers, but only the adhesion between the rubber composition and metallic material would be in question. For bonding the rubber composition to the metallic material, various methods have been proposed. So far as copper, copper alloys and other metals coated therewith are concerned, however, there is no need for using any adhesive, but it is possible to attain the required bonding merely by subjecting the assembly to be bonded to vulcanization treatment whereby sulphur combined in the rubber composition serves to realize the bonding between the metal and the rubber composition.
This method has been found to be simple and useful since it requires no troublesome treatments such as application of the adhesive, drying and the like.
The mechanism of this bonding has been considered to be due to Cu--S--C bonding formed by sulphur (S) bridging between copper (Cu) and carbon (C) in the rubber composition. Although there has been no experimental proof with respect to actual formation of this bridging, there are various theoretical views among which typical are "Rubber to Metal Bonding" authored by S. Buchan and published by Crosby Lockwood and Son Ltd., 1959; Yamato and Tanaka, Kogyo Kagaku Zasshi, 61, 252, 257 and 260 (1958) and Yamato and Tanaka, Nippon Gomu Kyokai Shi (J. of the Society of Rubber Industry, Japan), 32, 675 (1959).
The inventors have found that it is very important for attaining the desired vulcanization bonding to advance the vulcanizing reaction of the rubber in good balance of formation of said bonding or bridging in the boundary area between the metal and the rubber composition with simultaneous vulcanization of said rubber composition in the neighborhood of said boundary area, in view of the fact that adhesion therebetween is considerably lowered when the moisture is present in the rubber composition.
In order to attain the firm adhesion, according to said finding, the formation of the bonding Cu-S-C around the boundary area is necessary. However as suggested by Buchan, said formation should be advanced in good balance together with the vulcanization in said rubber composition without too great an amount of sulphur-sulphur bonding formation in said Cu--S--C bonding or an excess amount of Cu-S bonding therein. The reason why the presence of moisture when vulcanization bonding is carried out results in poor adhesion is explained by the fact that the moisture promotes the formation of too much copper sulphide around the boundary area while the vulcanizing reaction in the rubber composition and consequently formation of the strong bonding of Cu--S--C around the boundary area is harmed thereby.
Since so much time is required for mixing the rubber composition and assemblying the rubber composition with the metallic material to be subjected to vulcanization bonding, the unvulcanized rubber composition inevitably absorbs the moisture under the ambient conditions of high moisture and high temperature, which causes unsatisfactory bonding between the metallic material and the rubber composition so as to adversely affect the qualities of the product.
The inventors have made studies for finding improved method for vulcanization bonding without removing the moisture in question by using the desiccant in controlling the excessive reaction of copper with sulphur combined in the rubber composition which is apt to be advanced in the presence of the moisture as referred to above. As a result thereof it has been found that the addition of orthoboric acid or a metal borate to the rubber composition combined with sulphur and/or organic sulphur compounds can unexpectedly proceed with the bonding reaction between the metallic material and the rubber composition around the boundary area in good balance with the vulcanizing reaction in the rubber composition so that satisfactory adhesion can be attained despite the presence of a considerably greater amount of the moisture.